Methods for detecting focus states of photographing lenses include a phase difference detection method (deviation method).
In the phase difference detection method, light beams passing through an exit pupil of a photographing lens are divided in two, and the light beams divided in two are respectively received by a pair of sensors for focus detection. Then, the movement of the photographing lens required for focusing is obtained directly by detecting the deviations of output signals obtained in response to the amount of light received, that is, the deviations between the relative positions of the light beams in the dividing direction thereof. Thus, with one operation for accumulation carried out through the sensors for focus detection, the defocus amount and direction thereof are obtained, thereby allowing high-speed focusing operation. The following patent documents disclose techniques for providing a phase difference detecting function to an image sensor to eliminate the need for dedicated sensors for focus detection and achieve high-speed phase difference AF.
For example, Japanese Patent Laid-Open No. 2000-156823 discloses some light receiving elements (pixels) of an image sensor, in which a sensitivity area of a light receiving portion is located at a position shifted with respect to the optical axis of an on-chip microlens to provide a pupil divide function. Then, these pixels as pixels for focus detection are arranged at a predetermined interval between pixels for image sensing in which a sensitivity area of a light receiving portion is not located at a position shifted, to carry out phase difference focus detection. Furthermore, since the pixels arranged as pixels for focus detection are missing pixels for image sensing, image information for the pixels for focus detection is interpolated with the use of information obtained from peripheral pixels for image sensing.
Furthermore, Japanese Patent Laid-Open No. 2001-305415 describes light receiving portions in some pixels of an image sensor divided in two to provide a pupil divide function, and discloses a technique in which the outputs from the light receiving portions divided in two are individually processed to carry out phase difference focus detection, in which the outputs from the light receiving portions divided in two are added for use as image signals.
Moreover, Japanese Patent Laid-Open No. 2004-191629 discloses a technique for correcting the imbalance in the amount of light between first and second images, which is caused by vignetting of a photographing optical system when an image height is increased, in a technique for an image sensor provided with a phase difference detection function.
In Japanese Patent Laid-Open No. 2004-191629 described above, it is an object to correct the imbalance in the amount of light between first and second images, which is caused by vignetting.
On the other hand, problems caused by vignetting include not only the imbalance in the amount of light between first and second images, but also the movement of an image at a tilt due to a defocus state of first and second images with vignetting at a diagonal image height of an image sensor. In such a case, distance measuring errors may be caused depending on the subject.
The present invention has been conceived in light of the problems of the conventional art described above, and reduces detection errors caused by vignetting in a case in which an image sensor with a phase difference detecting function derived from pupil division in a pixel is used to carry out focusing.